Just one planet, Earth-sized and orbiting in less than a day.

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Hard as it may be to imagine, before 1988, we hadn’t discovered a single planet (or exoplanet) outside our own Solar System. We had good reason to believe they existed, but no one had ever observed one in practice. So when astrophysicists at the time created models of the formation of planets in general, they had only one data point to base these models on: the Solar System. Consequently, these models tended to predict systems that look a lot like the Solar System.

The systems we actually ended up discovering threw researchers for a loop. Many had massive gas giants orbiting extremely close to their stars, earning the name “hot Jupiters.” Things like warm Neptunes and super-Earths soon followed. It became clear that exosolar systems could have drastically different histories and formation processes.

In a new study, a team of researchers analyzed data from NASA’s Kepler spacecraft to learn more about the architecture of exosolar systems. Looking at 144 of them, the researchers identified a set that have a unique architecture: only one planet, an Earth-sized body orbiting extremely close to its parent star. The researchers estimate that systems like this account for about the same fraction of exostellar systems as the hot Jupiters do.

Whence the Singles?

Systems with only one known planet (sometimes called "singles") are relatively common, and indeed, there are more of them than predicted by models. The reason for this is unknown, so it called for a closer examination. The researchers reasoned that if they could identify populations of single planets with distinctive characteristics, they could potentially gain clues into the mechanisms that created them. It might even explain why there are more singles than expected.

To make their analysis more robust, the researchers only chose known planets that had a sufficiently low chance of being a false positive. By the end of their analysis, they concluded that less than two percent of the previously identified singles are false positives of one type or another. That means three planets or fewer.

Among groups of hot Jupiters and other systems, one group stood out. These systems had a single planet about the size of Earth, with an orbit that takes only a single day. Short orbits like these mean that these planets are extremely close to their stars—Mercury, the closest planet to the Sun, takes about 88 days to orbit. For this reason, the planets were dubbed “hot Earths.”

While the hot Earths all appear to be in singles, it’s difficult to rule out other planets existing in their systems that were missed by Kepler’s observation. This could happen if the planets are sufficiently small or far enough from the host star. So the researchers estimated the number of hot Earths that are in multis, and it turns out to be about 54 percent. That’s a lot, but it leaves plenty of hot Earths that are singles, meaning a unique population does exist.

Whence the hot Earths?

There are a few possible histories that could lead to the presence of a single hot Earth. One is that the hot Earths began their lives as hot Jupiters but lost their atmosphere when they got too close to their host star. The hard core would be all that remains. Indeed, the overall rate of these hot Earths is pretty similar to that of the hot Jupiters. If true, it would imply that there have been roughly twice as many hot Jupiters out there as currently estimated. The researchers suggest that this possibility could be tested by comparing the host stars of the hot Earths with those of the hot Jupiters; if they're similar, it may hint at a single origin.

Another possibility is that the planet forms in a disk with other planets, the innermost of which migrate inward early on, while the outermost planets move farther out. This would leave distinctive traces that could be looked for with ground-based telescopes. The last mechanism that's discussed is that these hot Earths may have formed as part of a multi-planetary system and later migrated inward due to interactions among the planets.

Of course, it's entirely possible that it’s something else entirely. Which makes understanding the origin of these systems that much more intriguing.